Das Jahr Null

Robert, his boyfriend Markus and his autistic brother Ferdi try to survive in a devastated, cold world, where zombies roam. Together with a group of other people they hide in an isolated mansion, but they all know they can't stay there forever. They need to get into the mountains in Austria, but they have only one small car. In addition to the rising tension, why does Ferdi sneak out at night in secrecy?